Stages of Sleep
For many centuries it was believed that sleep was a passive process that occurred for the sole reason of being restorative. 
It wasn’t until 1953 when Nathaniel Kleitman and Eugene Aserinsky, two pioneers of sleep research, demonstrated that sleep is actually composed of different stages that occur in a characteristic sequence. They were able to demonstrate this through the use of electroencephalographic (EEG) recordings taken from a number of healthy individuals. 
STAGES 1 to 4 = non-REM sleep
STAGE 5 = REM sleep
A typical 8 hours worth of sleep is comprised of several sleep cycles (generally 4-5), which alternate between non-REM and REM sleep. 
During the first hour of sleep, we gradually descend into the four successive stages of sleep.
The amount of time it takes to reach deep stage IV sleep from initial drowsiness is around one hour. 
The various stages of sleep can be defined by the EEG criteria they meet (as seen in the table 1.1). 
|Stage of Sleep||EEG Characteristics|
|Awake||(Beta activity) Frequency: 15-60 Hz, Amplitude: -30 μV|
|I (drowsy period)||Frequency: 4-8 Hz, Amplitude: 50-100 μV (theta waves)|
|II (light sleep)||Frequency: 10-12 Hz, Amplitude: 50-150 μV (spindles)|
|III (moderate to deep sleep)||Frequency: 2-4 Hz, Amplitude: 100-150 μV|
|IV (slow-wave sleep)||Frequency: 0.5-2 Hz, Amplitude: 100-200 μV (delta waves)|
|REM Sleep||Low voltage and high frequency (similar to activity seen when awake)|
What Is Non-REM Sleep?
The first four stages of sleep are classified as non-REM sleep. There are a number of physiological changes that occur during non-REM sleep, such as:
- Slow, rolling eye movements
- A decrease in muscle tone, body movement, heart rate, breathing, blood pressure, metabolic rate, and temperature. 
Each of these parameters reach their lowest values during stage IV of non-REM sleep. 
Stage IV of non-REM sleep is also known as slow-wave sleep. It is much more difficult to be woken up during slow-wave sleep, and for that reason it is considered as the deepest stage of sleep. 
Interestingly enough, slow wave sleep will usually only occur in the first and second cycles of sleep. 
What Is REM Sleep?
REM sleep is the stage of sleep that occurs after non-REM sleep has finished. The occurrence of REM sleep has been witnessed in all mammals, as well as a number of different birds and reptiles. 
As the name might suggest REM sleep is primarily characterized by the occurrence of rapid eye movements. Other characteristics of REM sleep also include pupillary constriction, paralysis of multiple large muscle groups, nocturnal penile erections, and the twitching of smaller muscles found in the fingers, toes, and middle ear. 
Sleep-related erections occur naturally during REM sleep in sexually potent men.  This occurrence has clinical significance as it can be used to determine whether a complaint of impotence has a psychological or physiological basis. [1, 2]
REM sleep only lasts for around 10 minutes, and once it has concluded the brain cycles back through the four stages of non-REM sleep again.
During REM sleep the electrical activity of the brain is very similar to that of someone who is awake. It is obvious that these two brain states are not the same though, for reasons such as the occurrence of dreams during REM sleep. 
There are several physiological changes experienced during this stage, including increased blood pressure, heart rate, and metabolic levels. 
- Purves, D, Augustine, G, Fitzpatrick, D, Hall, W, LaMantia, S, Mooney, R, Platt, M & White, L., 2017. Neuroscience. 6th ed. Sunderland, Massachusetts: Oxford University Press.
- Hirshkowitz, M. and Schmidt, M.H., 2005. Sleep-related erections: clinical perspectives and neural mechanisms. Sleep medicine reviews, 9(4), pp.311-329.